The present invention starts out from a simulation method and an apparatus of the species described in the main claim and/or the first apparatus claim. Generally, the attempt to substitute electro-acoustic circuits for partial systems of a human outer-ear model has been known heretofore, for example for simulating under headset reproduction conditions those ear signals which may occur under free-field sound propagation conditions when the sound arrives from random directions.
It has also been tried to provide electronic means representing a suitable average outer-ear transmission function for assumed directions of sound incidence--a procedure that can be described as sort of a directional mixer unit. For the purpose of building up and realizing such a directional mixer unit one may for example disregard the real mechanisms by which outer-ear transmission properties are generated--for example because these are unknown--, but determine instead empirically, by measurements taken on a number of test persons, an average outer-ear transmission function for a given, i.e. finite number of discrete directions of sound incidence. The outer-ear transmission functions so determined for the individual directions of sound incidence then permit switching of the "directional mixer unit" to the discrete directions, but has the drawbacks
(1) that there has been available to this date no averaging method for outer-ear transmission functions which safely leads to average transmission properties that are really adequate to the information receiver constituted by the "human sense of hearing";
(2) that a finite number only of different directions of sound incidence can be set; and that
(3) the input of cost and effort required for this work rises in proportion to the desired number of settable directions of sound incidence.
Another possibility to imitate by electronic means a so-called outer-ear simulator for simulating average outer-ear transmission functions in the time domain would for example consist in averaging and storing in a suitable manner transient responses that have been measured on test persons for all directions of sound incidence, a method which may require extremely large storage capacities, depending on the desired grid pattern. The output signal would in this case be the so-called convolution of the input signal with the two transient responses (for the left and the right ear) valid for the respective direction of sound incidence. Such real-time signal processing is, however, practically impossible because at least the signal processors presently available are capable of performing such signal proceesing only with considerable effort. For the same reason, the possibility of the so-called Fourier transformation of the input signal, followed by multiplication of the corresponding transmission functions and inverse transformation, must also be eliminated.
Although conventional mixer systems are in a position, by means of the so-called panoramic control, to distribute individual microphone signals to the two channels of a stereophonic transmission system so that when reproduced via two loudspeakers provided in the typical stereo arrangement, spatial distribution of aural phenomena between the two loudspeakers occurs (sum localization). However, this method has the disadvantages
(1) that the aural phenomena are present only within the space angle determined by the loudspeaker arrangement;
(2) that the height of the aural phenomena is frequently above the connection line between the loudspeakers and dependent on the position of the listener relative to the loudspeakers; and
(3) that during headset reproduction the aural phenomena occur as a rule in or on the listener's head because unusual and/or unnatural ear signals are offered to the hearing sense.
Due to these considerable problems and the high technical input required when ear signals are to be reproduced for many directions of sound incidence, there has been available to this day no practically useful form of realization of a so-called electronic artificial head or outer-ear simulator. Now, it is the object of the present invention to break new ground in this field and to provide an outer-ear simulator in the form of a so-called electronic artificial head which is capable, without greater expense, to realize infinite direction setting by amount and phase of all frequencies, giving due regard to the transmission properties of the human outer ear, and which in spite of the simplified structure operates with particularly high accuracy, i.e. which performs the simulation in such a manner that the transmission properties of the electro-acoustic circuit equivalent to the outer ear conform virtually identically to those of the human outer ear under free-field sound propagation conditions.